Vacuum hardening
A heat treatment process in a technical vacuum that enables oxidation- and decarburization-free heating and subsequent gas quenching of high-alloy materials.
Vacuum hardening is used when you need a clean, scale-free surface. In vacuum there is no oxygen, so the workpiece can neither oxidize nor decarburize. The component is heated in a vacuum furnace, quenched, and usually tempered right away. This process is ideal for high-alloy steels and tools that should not need post-machining after hardening. The parts come out of the furnace almost bright, with low distortion. But if you let air onto the hot part, you'll immediately get scale and the surface will become soft or cracked. So make sure the furnace is really tight and the vacuum is correct. The right hardening temperature and cooling strategy depend on the material – you have to check the data sheet, not guess.